In automobiles and various vehicles also categorized to the automobiles, a constant velocity universal joint capable of transmitting a rotation power at a constant velocity even if an angular displacement or an axial displacement occurs between those two shafts is arranged in a power transmission channel through which a driving force from an engine is transmitted to wheels. The constant velocity universal joints include a fixed type constant velocity universal joint that does not conduct plunging motion (axial displacement), and a slide type constant velocity universal joint that conducts the plunging motion. The fixed type constant velocity universal joints that have been widely known are a bar field type constant velocity universal joint (BJ) and an undercut free type constant velocity universal joint (UJ).
For example, as shown in FIGS. 23 and 24, the fixed type constant velocity universal joint of the UJ type includes an outer ring 3 that functions as an outer joint member in which a plurality of track grooves 2 are formed on an inner spherical surface 1 along the axial direction at regular intervals in the circumferential direction, an inner ring 6 that functions as an inner joint member in which a plurality of track grooves 5 that are paired with the track grooves 2 of the outer ring 3 are formed on an outer spherical surface 4 along the axial direction at regular intervals in the circumferential direction, a plurality of balls 7 that are interposed between the track grooves 2 of the outer ring 3 and the track grooves 5 of the inner ring 6 to transmit torque, and a cage 8 that is interposed between the inner spherical surface 1 of the outer ring 3 and the outer spherical surface of the inner ring 6 to hold the balls 7. A plurality of pockets 9 that receive the balls 7 are arranged in the cage 8 along the circumferential direction.
Also, each of the track grooves 2 of the outer ring 3 has a circular arc portion 2a at a back side, and a straight line portion 2b at an opening side. Each of the track grooves 5 of the inner ring 6 has a straight line portion 5a at a back side, and a circular arc portion 5b at an opening side. The center curvature O1 of the track grooves 5 of the inner ring 6 and the center curvature O2 of the track grooves 2 of the outer ring 3 are offset with respect to a joint center O at regular distances F in opposite directions along the axial direction, respectively.
In recent years, in order to downsize the universal joint, eight balls are disposed as shown in FIG. 24 (Patent Document 1). In this case, when it is assumed that the cage thickness at the pocket center position of the cage is tCAGE, and the pitch circle radius of the balls when the operating angle is 0° is PCRBALL, tCAGE/PCRBALL that is the ratio of the cage thickness and the pitch circle radius is set to about 0.11 to 0.19.
Also, in the conventional UJ type constant velocity universal joint, as shown in FIG. 26, for the purposes of reducing the universal joint in size and weight, the offset angles θTRACK of the track grooves which are defined by lines that connect the center curvatures O1 and O2 of the respective track grooves 2 and 5 of the inner and outer rings 3 and 6 and the centers Q of the torque transmission balls 7, and lines that connect the centers Q of the torque transmission balls 7 and the joint center O are set in a range of 4°≦θTRACK≦6° (Patent Document 2).
In the conventional UJ type constant velocity universal joint disclosed in Patent Document 2, the center curvature O10 of the inner spherical surface 1 of the outer ring 3 (the center curvature of an outer spherical surface 8a of the cage 8) and the center curvature O20 of the outer spherical surface 4 of the inner ring 6 (center curvature of an inner spherical surface 8b of the cage 8) are also offset across the joint center O at regular distances in the axial direction, respectively. Then, the offset angles θCAGE of the cage 8 which are defined by lines that connect the center curvatures O10 and O20 of the inner and outer spherical surfaces 8a and 8b of the cage 8 and the center Q of the torque transmission balls 7, and the lines that connect the centers Q of the torque transmission balls 7 and the joint center O are set in a range of 0°<θCAGE≦1°. In this way, because the cage 8 is set to be very small in the offset angle θCAGE, the cage 8 is formed with the substantially same thickness.    Patent Document 1: JP 3859264 B    Patent Document 2: JP 2005-337304 A